The present invention relates generally to aircraft gas turbine engines, and, more specifically, to vectoring exhaust nozzles therefor.
A typical high performance, augmented gas turbine engine includes a varying area converging-diverging exhaust nozzle which is axisymmetric about a longitudinal or axial centerline axis. The nozzle includes a plurality of circumferentially adjoining primary exhaust flaps joined in turn to a plurality of circumferentially adjoining secondary exhaust flaps. The secondary flaps are joined by corresponding outer compression links to a common stationary casing also supporting the primary flaps.
This assembly is articulated in the manner of four-bar linkages to vary exhaust flow area, designated A8, at the nozzle throat between the primary and secondary flaps, and for varying the flow area of the nozzle outlet, designated A9, at the downstream end of the secondary flaps. Suitable linear actuators such as hydraulic actuators are circumferentially spaced apart around the casing and have respective output rods joined to the nozzle for pivoting the primary flaps to control the throat area and in turn control the outlet-to-throat area ratio.
In order to increase the maneuverability of aircraft powered by augmented gas turbine engines, vectoring exhaust nozzles are being developed. In U.S. Pat. No. 4,994,660, assigned to the present assignee, an Axisymmetric Vectoring Exhaust Nozzle (AVEN.RTM.) is disclosed. In this type of nozzle, a primary actuation ring surrounds corresponding cams on the outboard surfaces of the primary flaps and is operatively joined to a plurality of primary linear actuators which control its axial position perpendicular to the axial centerline axis of the nozzle. The outer links in this nozzle are joined to a secondary actuation ring which in turn is joined to a plurality of secondary linear actuators mounted to the casing.
During operation, axial translation or slide of the primary ring controls the pivoting of the primary flaps and in turn the nozzle throat area. The secondary ring may also slide axially to independently control pivoting of the secondary flaps and in turn control both the outlet area and the area ratio. Furthermore, the secondary ring may be tilted in space to effect pitch or yaw, or both, in the secondary flaps to effect nozzle vectoring in which the engine exhaust is discharged at a slight angle from the engine centerline axis as opposed to coaxially therewith as in conventional non-vectoring exhaust nozzles.
Since the secondary flaps are vectorable they substantially increase the complexity of the nozzle design and its implementation. For this reason, many additional patents have been granted on various features of the AVEN.RTM. exhaust nozzle in behalf of the present assignee. These patents relate to both the mechanical details of the nozzle and the control systems therefor.
Since a plurality of circumferentially adjoining secondary flaps are utilized in the nozzle, suitable inter-flap seals must also be provided for preventing flow leakage between the flaps as the flaps are positioned through a suitable range of vectoring. This range, however, is limited to avoid inter-flap flow leakage or undesirable distortion of the various components.
Furthermore, the control system for the vectorable nozzle is being developed for a digitally programmable controller to process the required data in real time and control the actuators in feedback closed loops. The nozzle controller typically includes limiting values to prevent excess vectoring of the nozzle within the mechanical capabilities of the nozzle components. And, the nozzle controller must be sufficiently fast to process the required data in real time for the extremely fast maneuvering of the nozzle and the aircraft being powered therewith.
In order to obtain sufficient real time processing capability of the vectorable exhaust nozzle, the secondary actuators have been equiangularly spaced apart from each other symmetrically about the vertical and horizontal axes. Nozzle yaw is defined by the angular rotation of the secondary flaps about the vertical axis. And, nozzle pitch is defined by the angular rotation of the secondary flaps around the horizontal axis, which is perpendicular to the vertical axis. Combinations of pitch and yaw are also possible for providing full 360.degree. vectoring capability of the nozzle.
The symmetrical orientation of the secondary actuators significantly simplifies the control algorithms for converting pitch and yaw commands into the required movement of the secondary actuation ring for in turn positioning the secondary flaps. In one simple control scheme, actuators above and below the horizontal axis are driven in proportional magnitude both forward and aft to effect nozzle pitch. Similarly, the actuators may be driven symmetrically on opposite sides of the vertical axis both forward and aft to effect nozzle yaw.
In view of the fixed, predetermined, and symmetrical relationship of the actuators on the secondary actuation ring, simple trigonometric relationships for determining actuator stroke to effect pitch and yaw were previously developed. This greatly simplifies the control algorithms and allows the use of known trigonometric functions for specific angles associated with the actuators. The fixed trigonometric functions may therefore be represented in the control algorithms by the numerical values thereof, between zero and one, and suitably stored in memory for subsequent use.
However, since the exhaust nozzle must be mounted in corresponding engine bays in various types of aircraft, space limitations may prevent the symmetrical mounting of the secondary actuators, and the attendant simplification of control algorithms. If the actuators are not symmetrically attached to the secondary actuation ring, or if the attachment changes from design to design, the simple control algorithms will be ineffective in controlling vector operation.
Furthermore, more than three actuators may be used for the secondary actuation ring which requires precise control of the fourth or more actuators to prevent opposition with the initial three actuators which define the plane of the secondary ring. In some designs, it may be desirable to employ two redundant secondary actuator systems, with each system having three actuators. The six actuators must therefore be controlled in unison to prevent opposition load therebetween and to ensure that all the actuators operate synchronously. Two redundant systems reduce the likelihood of symmetrical orientation of all of the actuators and a resulting simplified control system therefor.
Accordingly, it is desired to provide a generic control system for the vectoring actuators in an exhaust nozzle which is operable regardless of the circumferential location of the individual actuators, yet, at the same time, reduces the complexity of the required control algorithms therefor, and reduces the need for repetitive calculations which would otherwise increase processing time, which is limited in real-time control of the exhaust nozzle.